1. Field of the Invention
The present invention relates to the design of an attenuation reduction grounding structure of high-frequency signal transmission lines of a circuit board, and in particular to an attenuation reduction grounding structure of differential-mode signal transmission lines of a flexible circuit board, which comprises at least one vie hole extending through a thin metal foil layer, a bonding material layer, and a covering insulation layer formed on the flexible circuit board and corresponding to a conductive contact zone of a grounding line, with the via hole being filled with a conductive paste material layer to electrically connect the thin metal foil layer to the conductive contact zone of the grounding line.
2. The Related Arts
Most electronic devices are provided with circuit boards or flexible flat cables to position all necessary circuit components and connectors and realize transmission of electronic signals. In the manufacture of a circuit board, wires are laid on a surface of a substrate to form an extended signal transmission line for transmission of the electronic signals.
A flexible substrate of a flexible circuit board comprises a plurality of differential-mode signal lines formed on a surface thereof to be parallel and spaced from each other by a predetermined spacing distance. The differential-mode signal lines are arranged in pair and generally in combination with an adjacent grounding line. A covering insulation layer is formed on a surface of the substrate and covers surfaces of the differential-mode signal lines and the grounding line. To achieve an effect of shielding, a shielding layer is commonly formed to cover a surface of the insulation layer and is grounded.
In transmission of a differential-mode signal through the differential-mode signal transmission lines, if the grounding line has a poor grounding effect, then the differential-mode signal lines regard the shielding layer and the grounding line that is actually grounded as two grounding surfaces. This leads to interference of signal and affects the performance and reliability of transmission of high-frequency signals.
Further, the differential-mode signal lines are generally made of a copper foil material or a composite material and are of a structure having a cross-section of a substantially flat configuration. Ideally, the differential-mode signal lines each have opposite sidewalls that are normal to the surface of the flexible substrate. However, in a real structure, the two sidewalls of a differential-mode signal line show a slope shift amount (namely being non-vertical sidewalls), so that a left top corner and a right top corner of the differential-mode signal line each form a top angle structure and a left bottom corner and a right bottom corner of the differential-mode signal line each form a bottom angle structure.
In the transmission of an electronic signal having a relatively low frequency, the angle structures of the differential-mode signal line do not cause any problem. However, in the transmission of a high-frequency signal, the top angle structures and the bottom angle structures may cause attenuation of the signal. Such a situation leads to problems of poor signal transmission reliability, poor impedance control, and signal interference in the transmission of a high-frequency signal through the high-frequency signal transmission lines that are commonly used in the contemporarily available electronic devices.